This module takes a number of parameters as input, such as
initial string, change string,the angle of the branch, and the
size(depth of recusive) of the plant. The output is a field,
consisting of segments,that build the tree. Using Data Explorer,
the output combined with other objects and rendered. For ease of
use, the module is an Outboard module.

The basic idea of turtle interpretation described by Prusinkiewicz[2] is given
below. A state of the turtle is defined as a triplet state
(X,Y,Alfa), where the Cartesian coorfinates(x,y) represent the
turtle's position , and the angle Alfa ,called the heading ,
is interpreted as the direction in which the turtle is facing.
Given the step size N and the angle increment angle Delta ,the
turtle can respond to commands represented by the symbols :
F , + , and -.

However, the three symbols can only generate 2 dimensional
graphic trees. If we want to generate 3 dimensional graphics,
these two operators are not enough.
Therefore ,for 3D graphics, We have to change the tutle's state
to (X,Y,Z,Angle_U,Angle_L,Angle_H).

In addition, there are seven operators to deal with the freedom
of the 3 dimensions graphics space, such as turn left or right,
pitch down or up,and roll left or right, and turn around.
All these operators are used to generate the graphics not only
in flat plant but also in 3D space.

Angle_U,Angle_L and Angle_H are the three free angle in space,
and we increase the number of operators to seven symbols:
+ , - , & , ^ , < , > , |. In 3D,the key concept is to represent
the current orientation of the turtle in space by three vectors
H,L,U, indicating the turtle's heading, the direction to the
left and the direction up.These vectors have unit length, are
perpendicular to each other ,and satisfy the equation H cross L = U.
Rotatations of the turtle are then expressed by the equation:

[H' L' U' ] = [ H L U]R

where R is a 3*3 rotation matrix. Specifically, rotations by
angle Alfa about vectors U ,L ,H are represented by the
following matrices:

In this module, all plants generated by the same deterministic L-system are
identical. To extend the module function, we can use the concept of the probability, we can generate
different tree graphics. It is necessary to introduce specimen-to-specimen
variations that will preserve general aspects of a plant but will modify its details.
For example, F-->F[+F][F] (P=0.33) and F-->F[-F][F](P=0.67).
Moreover, if we introduce more Variables, not only F variable
,for example, the following variables: A,B,C,D,E,F...., we will generate more
complicate trees in space.